1. Field of the Invention
The present invention is directed to a solution which makes it possible to make high-quality recordings of the anterior and/or posterior segments of the eye as an individual image or also as a sequence of images without increasing the radiation load on the eye to be examined.
2. Description of Related Art
According to the known prior art, flash lamps are used in fundus cameras and slit lamps to illuminate the relevant areas of the eye for achieving recordings of correspondingly high quality. Observation and documentation are carried out in this case by means of an electronic camera.
Depending on the area of the eye to be illuminated, the exposure must be controlled exactly, because too much light leads to over-illuminated images and too little light leads to images with poor contrast and high noise. Optimal illumination data are made even more difficult to achieve in that these data vary for the different recording modes (monochrome recordings, color recordings or fluorescence recordings). The illumination intensity can vary owing to variations in the spectral composition of the illumination light, the size of the ring illumination, or the width of the slit illumination.
Additional factors influencing optimal exposure, aside from the individual differences between the illuminated objects (pupil size, reflection factor of the retina, cornea and lens), include individual differences in opthalmological instruments (optical characteristics of the components used, differences in component assemblies, tolerances, aging, soiling, etc.).
Not least of all, individual user requirements such as, e.g., gradual overexposure or underexposure should be taken into account in the optimal exposure or, more precisely, in achieving the optimal signal-to-noise ratio for the electronic image recording.
Solutions to these problems are known in the prior art in which multiple recordings are made by the user and the exposure time is successively, individually adapted. The disadvantage consists in the high, repeated radiation load on the patient's eye. In other known solutions, a portion of the reflected light is coupled out of the observation beam path, integrally summed, and used for controlling the flash. Otherwise, the user accepts recordings which are not optimal.
DE 10 24 2851 A1, DE 10 2004 011 906 A1, and EP 0 512 508 B1 describe solutions for controlling the flash exposure in photographic cameras. The exposure is measured during the recording through the objective (TTL=Through The Lens) and works with a pre-flash in the visible range and associated electronic evaluation. The light measurement is carried out zone by zone, a photodiode being provided for the central measurement and an array for measuring the ambient light. The pre-flash is automatically repeated as needed. Apart from the visible pre-flash and the evaluation merely zone-by-zone in partial areas, it is also disadvantageous that a photodiode and an array of photodiodes are additionally required.
DE 31 39 547 C2 and DE 33 32 835 C2 describe a solution for exposure control in a photographic camera or a device for advance information about a photographic flash exposure which work with a pre-flash in the visible range and associated electronic evaluation. The visible pre-flash and merely zonewise evaluation requiring an additional photodiode are disadvantageous.
DE 33 47 872 C2 describes a photographic camera with spot exposure measurement and integral exposure measurement in which the light measurement is carried out zone by zone. In addition, a photodiode is provided for the center spot measurement and a second photodiode is provided for measuring the ambient light. It is disadvantageous that the evaluation is only carried out zone by zone.
A pre-flash method in which an integral light measurement is carried out with only one photocell is described in DE 690 26 826 T2. It is disadvantageous that the evaluation which is only carried out point by point requires additional photodiodes or an array of photodiodes. Further, with integral measurement of the light energy very small areas of the recording can be over-illuminated already when the predominant, residual image area of the recording is dark. Therefore, an integral measurement of the light intensity while the picture is being taken is unsuitable particularly for slit recordings.
Solutions of the kind mentioned above with visible pre-flashes and/or multiple recordings have the disadvantage of very high radiation loading of the patient's eye.
In order to give calculated light outputs accurately, flash lamps with reproducible flash output are required and must be controlled by an elaborate flash control with switch-on and switch-off delays having an accuracy in the range of about 50 μs.
Since the pre-flash and principal recording are generally evaluated separately, a plurality of light-sensitive sensors are required.
For example, in the system for flash photography described in U.S. Pat. No. 6,094,536 A, a separate light-sensitive sensor is also used for light measurement. The solution for determining the data for a main flash is based on the evaluation of a light measurement of a first flash of constant intensity and a light measurement of the environment without flash illumination. In this way, not only the amount but also the constant intensity of the flash output is controllable over a given period of time.
A fundus camera with a flash lamp for realizing recordings of an eye is described in U.S. Pat. No. 5,557,321 A. The proposed system has two image recording devices of different sensitivity. One image recording device carries out low-resolution overview recordings and the other image recording device carries out high-resolution detailed recordings. The light output emitted by the flash lamp is matched to the appropriate sensitivity. This solution has the drawback that two photographic systems are required, which makes the overall construction of the fundus camera more complicated and expensive.